1. Field of the Invention
The invention relates to a novel process for the catalytic oxidation of pseudocumene to trimellitic acid.
2. Background Art
Trimellitic acid is employed in the plastics industry as an intermediate for the preparation of synthetic resins, plasticisers, etc. Frequently, the trimellitic acid is reacted directly via a dehydration step to give trimellitic anhydride which, in turn, is widely used as an intermediate in the plastic industry, in particular in the preparation of polyesters.
Numerous catalytic oxidation processes are known in order to react pseudocumene to give trimellitic acid. Thus, it already follows from U.S. Pat. No. 2,833,816 to oxidize polyalkylaromatic compounds, such as, trimethylbenzenes, in the presence of cobalt, manganese, cerium and bromine to give the corresponding polycarboxylic acids. However, it was only possible to achieve a yield of 52.5 mol percent for the reaction of pseudocumene to give trimellitic acid. According to U.S. Pat. No. 3,683,016, it was possible to increase this yield by adding a cobalt/manganese/cerium/bromine catalyst stepwise to the oxidation of pseudocumene. As Comparison Example A below shows, it was possible to increase the yield considerably even if the values of the process according to the invention of this application were not attained. A serious disadvantage, however, was the low selectivity of this known process, which led to an increased secondary product formation and thus showed itself by a considerable discoloration of the product in the further reaction to give the anhydride. It was also attempted to replace cerium by zirconium. It thus follows, for example, from U.S. Pat. No. 4,755,622, to react pseudocumene to give trimellitic acid or further to give the anhydride using a cobalt/manganese/ zirconium/bromine catalyst. As Comparison Example B below shows, the yield of the known process is only slightly below the values of the process according to the invention of this application. A considerable disadvantage is again the selectivity, which leads to relatively high secondary product formation and thus leads to considerable discoloration of the product in the direct further reaction of the acid to give the anhydride.
Since, as mentioned above, trimellitic anhydride is employed for the preparation of polyesters, an inferior quality of the starting materials considerably influences the properties of the polyester prepared therefrom.